Principles of naval engineering - Historic Naval Ships Association

PRINCIPLES OF NAVAL ENGINEERINGtransmitter (Fl). The difference in the twosignals put into the differential relay (F4-1)causes a decreased output pressure to betransmitted from the differential relay to thefeed water Standatrol (F4-2). The feed waterStandatrol therefore sends a decreased signalthrough the feed water selector valve (F5) tothe feed water flow control valve, causingthe valve to begin to close.Let us examine this point more closely.The steam demand has increased but the feedwater flow control valve is closing . Why? Becauseit is necessary to compensate for swell—the momentary increase in the volume of thewater that occurs when the firing rate is increased.As swell occurs, the pneumatic signalfrom the drum water level indicating transmitter(F3a) increases. As a result, the outputpressure of the feed water Standatrol (F4-2)begins decreasing even more rapidly, closingdown on the feed water flow control valve (F6a)and further restricting the flow of feed waterto the boiler.As the feed water flow decreases, there isa proportional drop in the pneumatic pressurefrom the feed water flow transmitter. The effectsof this pressure decrease are felt slowly,however, because of the restricting action of thebleed valve in volume chamber Fla.As the steam drum water level begins todrop, there is a proportional decrease in thepneumatic pressure from the drum water levelindicating transmitter (F3a). At the same time,the bleed valve in volume chamber F2c is decreasingthe pneumatic signal between the steamflow transmitter (F2) and the transient compensatingrelay (C4a3) and increasing the pressurein another chamber of the compensating relay.The effect of this bleed valve action is to balancethe inputs to the two chambers of the compensatingrelay so that the compensating relay outputpressure is now equal to the pressure it is receivingfrom the steam flow transmitter. Inother words, the reversing action of the transientcompensating relay (C4a3) has been stopped,and the compensating relay is now transmittinga pneumatic signal that is exactly the same as thenew (and higher) steam flow signal it receives.The increased loading pressure from thetransient compensating relay (C4a3), togetherwith the decreased loading pressure from thefeed water flow transmitter (Fl), increases theoutput pressure of the steam flow-water flowdifferential relay (F4-1). The increased outputpressure of F4-1 reverses the action of the feedwater Standatrol (F4-2) and causes its outputto increase, thus opening the feed water flowcontrol valve wider and allowing more feedwater to flow to the boiler.When the feed water flow is equal to thesteam flow, and when the steam drum waterlevel has returned to normal, the systemstabilizes and the output of the feed waterStandatrol (F4-2) stays at the higher valuewhich will maintain the new and higher rateof feed water flow.A similar (but of course reversed) seriesof events occurs when there is a decrease insteam demand. The first effect of the decreasedsteam demand is a wider opening ofthe feed water flow control valve to compensatefor shrink— that is, the decrease in thevolume of the boiler water that occurs whenthe firing rate is reduced. The final effect isa smaller opening of the feed water flow controlvalve and a reduced flow of feed water tothe boiler.MaintenanceTo ensure trouble-free operation of thecontrol system, it is important that the systembe properly maintained and calibrated atall times. Maintenance and calibration shouldbe conducted in accordance with the PlannedMaintenance Subsystem of the 3-M System.An example of maintenance actions and teststo be conducted on an Automatic Control Systemare shown in figure 11-42. Particularemphasis should also be placed on the use ofmaintenance, repair, calibration proceduresfound in the applicable manufacturer's technicalmanual. If each component is kept in a properlymaintained and adjusted condition, the need fora general overhaul or major recalibrationof thecontrol system will be minimized.NOTE: When checking the adjustments andcalibration of any component of the controlsystem the settings should not be changed exceptunder the supervision of "QUALIFIED"maintenance personnel. It is also extremelyimportant, when making adjustments to thecontrol system, that the person doing the workknow the effect adjustments have on the operationof the entire control system. In otherwords only "QUALIFIED" personnel should beallowed to perform maintenance, repair, andcalibration of any automatic control systemcomponents.318